1. Field of the Invention
The present invention relates to integrated circuits; and, in particular, the present invention relates to integrated circuit for driving a video display.
2. Discussion of the Related Art
A monitor used in a computer system is often equipped with a on-screen menu system to provide an “on-screen display” (“OSD”) of one or more menus, showing current settings or functions of the monitor. When such a menu is displayed, an OSD window is typically created which interrupts or overlays at least a portion of the visible screen area of the video display. To achieve this overlay function, the video preamplifier is provided with a blanking capability which, when activated, inserts the OSD data in place of the video data while the video data is blanked.
FIG. 1 is a block diagram of a single-channel video preamplifier 100 in the prior art. As shown in FIG. 1, an analog signal representing one of the three color input signals or “channels” (R, G or B) is provided at terminal 101 as an input signal to video preamplifier 100. This analog signal is then amplified by an input amplifier 102. The amplified signal output of input amplifier 102 is then attenuated by video contrast attenuator 103 in accordance with a contrast attenuation signal at terminal 113 under user control. This contrast attenuation signal is typically provided externally and is common to all three channels. The contrast-attenuated signal is then adjusted in drive attenuator 104 in accordance with a channel-specific control signal at terminal 114. The output signal of drive attenuator 104 at terminal 116 is then amplified in output amplifier 105. The output signal at terminal 115 of output amplifier 105 is limited by an output signal of amplifier 106 (“clamp comparator”) in response, when an external clamp signal at terminal 107 is asserted, to an externally imposed cutoff voltage at terminal 108. An external clamping capacitor (not shown) is provided coupled between terminal 109 and a reference voltage, typically ground or a supply voltage, to impose at terminal 109 a DC offset voltage to the output video signal at terminal 110. The attenuated video signal at terminal 116 is summed in amplifier 111 with the DC offset voltage at terminal 109, to provide at terminal 110 a video output signal. This video output signal at terminal 110 can be grounded by a blanking signal asserted at terminal 112. When the blanking signal at terminal 112 is asserted, i.e. the video output signal at terminal 110 is grounded, OSD data generated by an external OSD integrated circuit (not shown) is inserted at terminal 110 to provide the OSD overlay.
One disadvantage of video preamplifier 100 discussed above results from the timing and delay limitations of the blanking system, so that the quality of the on-screen display within the OSD window is compromised. In addition, in a typical high-frequency video signal path, a complex technique is necessary to inject the OSD data into terminal 110.
FIG. 2 is a block diagram of another prior art video preamplifier 200. To simplify description and to facilitate identification, like elements in video preamplifiers 100 and 200 are provided the same reference numerals. As shown in FIG. 2, in addition to the R, G or B input video signal at terminal 101, preamplifier 200 accepts also a corresponding R, G or B OSD input signal at terminal 209. In preamplifier 200, the video input signal received at terminal 101 is assumed to have a predetermined maximum dynamic amplitude of one volt, measured peak-to-peak. Thus, a 2-volt reference voltage (“black DC”) is provided along with amplifiers 202 and 203 to clamp, when the signal at clamp gate terminal 107 is asserted, the input video signal at terminal 101 to a 2-volt DC offset or reference voltage. The video input signal of terminal 101 is thus provided at terminal 206 as an AC 700 mV peak-to-peak video signal superimposed on the DC offset voltage. The OSD input signal 209, originally at 0 to 4 volts peak-to-peak, is likewise amplified and provided with a DC offset voltage by amplifier 204, to provide an OSD input signal at terminal 207, also as an AC video signal having a maximum amplitude of 700 mV peak-to-peak. Under control of a select signal at terminal 205, a fast commutator or switch 201 is provided to select between the OSD input signal at terminal 207 and the video input signal at terminal 206. As in preamplifier 100 of FIG. 1, video contrast attenuators 103a and 103b are each provided for attenuating the corresponding one of the input video signal at terminal 206 and the input OSD signal at terminal 207. The remainder circuitry in preamplifier 200, i.e. drive attenuator 104 and amplifiers 105, 106 and 111, function in the same manner as the corresponding elements in FIG. 1 described above.
While video preamplifier 200 overcomes both the problem of poor quality on-screen display and the problem of complex OSD data insertion in video preamplifier 100, video preamplifier 200 is a more complex circuit having a higher power dissipation, a larger chip size and a higher pin count. Preamplifier 200 has a higher pin count because five pins are required per channel; namely, an R, G or B video input pin, an OSD input pin, an OSD/RGB select pin, a contrast pin and drive attenuation pin are required.